An apparatus of the above-mentioned kind is known from German patent DE 35 03 088 C2 owned by the assignee of this application. The apparatus described therein uses positioning units whose piston rods are provided with U-shaped pressure pieces. For the transfer of pressure force, the shank of each pressure piece lies on a box-shaped pressure plate which distributes the force areally and transfers it to an equalizing mat. Each box-shaped pressure plate is so supported in a carrier of the pressing beam that it can be inclined in different directions, yet is secured with respect to rotation about a vertical axis perpendicular to the work plane of the grinding belt. Moreover the support of the box-shaped pressure plates permits a lifting and lowering of them together with the piston rods. Although the known apparatus operates satisfactorily, it has nevertheless various disadvantages. For example, the support of the box-shaped pressure plates is technically expensive. Also the assembly of the pressing beam with the plurality of controllable positioning units is complicated. Likewise the maintenance or possible disassembly of the pressing beam for repair purposes is technically expensive.
From DE-OS 26 34 829 a pressing beam for an endless circulating grinding belt of a belt-grinding machine whose equalizing mat is held by a C-shaped bracket is known. Arranged inside of this bracket is a pillow-shaped body which can be acted upon by pressurized air and which is available to press the equalizing mat to the grinding belt with the required pressing force.